arch_init.h: Add QEMU_ARCH_HEXAGON

arch_init.h: Add QEMU_ARCH_HEXAGON

When Hexagon was added we forgot to add it to the QEMU_ARCH_*
enumeration.  This doesn't cause a visible effect because at the
moment Hexagon is linux-user only and the QEMU_ARCH_* constants are
only used in softmmu, but we might as well add it in, since it's the
only architecture currently missing from the list.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Taylor Simpson <tsimpson@quicinc.com>
Message-id: 20210730105947.28215-6-peter.maydell@linaro.org